1. Field of the Invention
The present invention relates generally to the compressed air and gas industry. More particularly, the present invention relates to the use of an apparatus for the removal of water from compressed gas streams and a method of making the device. Most particularly the present invention relates to an improved membrane module for use in such systems and a novel method of manufacture thereof.
2. Discussion of the Related Art
Membrane gas dehydration modules are known in the industry and art as effective means for removing water vapor from compressed gas streams. One of the common gas streams that is dehydrated with membranes is air. Several examples of membrane air dryers can be found in U.S. Pat. No. 4,783,201 by Rice; U.S. Pat. No. 5,002,590 by Friesen; and U.S. Pat. No. 5,067,971 by Bikson.
A common feature of membrane air dryers is that a certain amount of the compressed feed air is lost in the process of dehydration because of its use as the sweep gas. This air is commonly known in the art as the sweep or purge. Several methods exist in the art for achieving and controlling the purge amount. U.S. Pat. No. 4,783,201 specifically controls the air permeability of the fibers so that only a predetermined amount of compressed feed air permeates the membrane. In U.S. Pat. No. 5,160,514 an external valve is used to expand out a portion of the dried product air and reintroduce it into the shell side of a lumen fed hollow fiber module. Additionally, methods exist in the art where the purge air is supplied to the shell side of the fibers via holes or orifices placed internally in the potting compound on the product end of the module. In this case, the sweep air exits the orifice or hole at near sonic velocity into the shell side of the module. It is known in the art that in order to achieve efficient use of the membrane area and the purge air the distribution of the purge air is critical, and optimal performance is achieved if the purge air is uniformly distributed.
The membrane air dryers known in the art generally function by contacting one side of a semi-permeable membrane with a pressurized wet feed stream. The membrane is chosen such that it preferentially allows water to permeate faster than the feed gas that is being dehydrated. A portion of the dried gas known in the art as the sweep is depressurized and contacts the other side of the membrane and acts to drive away the water moisture that has permeated the membrane. While the form of the membrane may be either flat sheet, or hollow fiber, most commercially available membrane air dryers are made with hollow fibers. Additionally, for typical plant installations, where the air pressure does not exceed 200 psig, the feed gas is introduced to the lumen of the fibers, and the sweep air is introduced to the shell of the module and contacts the outer surfaces of the fiber. Typically this sweep air flows generally counter current to the flow of the compressed air stream.
Since it is desirable to have a uniform sweep rate many methods have been developed to control and introduce the sweep air into the module. Specifically, many methods have been developed to control and introduce the sweep air into the module where a hole or orifice located internally in the potting compound of one end of the module (typically the product end for counter current flow) is used to provide sweep air.
While this method of sweep introduction is viable for many applications, it suffers from several drawbacks, especially as the orifice size increases. These drawbacks relate to the jet-like nature of the air flow exiting the orifice or tube, and the possibility of degradation of the fibers in the module. Thus, those skilled in the art continue to search for a satisfactory solution as to how to control and introduce the sweep air into the module, obtain a uniform sweep flow distribution, and at the same time have no fiber degradation.
The present invention solves the problems present in the related art by replacing internal orifices or needles typically used in hollow fiber modules with an orifice to serve as a sweep inlet, and a sweep diffuser placed a short distance from the orifice. A novel method of manufacturing such a module is also disclosed.
In one embodiment of the present invention a membrane module is provided having an open channel extending from the product end of the membrane module on the shell side of the fibers with a porous diffuser at one end of the open channel, and an orifice at the other end of the channel.
In another embodiment of the present invention, a method of manufacturing a module of the foregoing nature is provided comprising the steps of:
a) inserting a plug into a one ended porous tube, inserting the combination so produced into the end of a bundle of fibers before potting, potting at least one end of the membrane module to fix in place the combination of the plug and porous tube, cutting the end of the potted module for sufficient distance to open up the lumen side of the fibers, removing the plug from the porous tube, and inserting an orifice into the open channel proximate the end of the module.
Thus, it is one of the objects of the present invention to provide an improved method of sweep introduction for a membrane module.
Another object of the present invention is to provide an improved method of introducing sweep gas into the shell side of a membrane module which provides for a more uniform sweep rate.
A further object of the present invention is to provide an improved method of the foregoing nature which eliminates or reduces fiber degradation.
A further object of the present invention is to provide a membrane module wherein less of the shell area of the module is bypassed by the sweep flow.
A still further object of the present invention is to provide an improved membrane module wherein a porous plug and an orifice are used in place of a sweep needle to introduce the sweep gas into the shell side of the module.
Further objects and advantages of the present invention will be apparent from the following description and appended claims, reference being made to the accompanying drawings forming a part of the specification, wherein like reference characters show corresponding parts in the several views.